1. Field of the Invention
The present invention relates to water purification, and particularly to a sunlight active composite photocatalyst for water purification that provides a composite photocatalyst for single-step removal of chemical and biological contaminants from water.
2. Description of the Related Art
New and improved methods of providing clean water must be identified in order to meet the demands of a growing world population. Efficient methods for water decontamination need to be explored, as existing multi-step water treatment technologies have been inefficient and uneconomical. The majority of water pollutants are hazardous chemical compounds and pathogenic microorganisms. The key chemical contaminants found in water generally include a variety of organic compounds, such as complex colored substances and stable phenol derivatives. These compounds are either released from the effluents of the chemical/petrochemical industry or generated in situ during chlorination and ozonization treatments for removal of biological contaminants. This class of secondary pollutants, due to their chemical stability and complex molecular structure, is difficult to remove by conventional techniques and typically requires in situ generation of highly energetic oxidation species for complete mineralization. In addition, their degradation by conventional procedures leads to the formation of another stream of secondary pollutants, which further aggravates the situation.
In the field of photo catalysis, titanium dioxide (TiO2) has generally been the most widely studied photocatalyst. The efficiency of a photocatalytic process may be measured in terms of its ability to absorb and harvest the absorbed photons. While TiO2 has been used for air and water decontamination, TiO2 absorbs only 3% of the solar energy due to a wide bandgap of 3.2 eV, which imposes a serious restriction on its use in sunlight. In fact, many conventional photocatalytic processes suffer from shortcomings related to low photon absorption capability due to wide bandgaps and high recombination rate of charge carriers. Almost all of the active photocatalysts (such as TiO2, ZnO, WO3 and NiO) suffer low activity in the sunlight, either due to wide bandgap, i.e., ≧3.0 eV, or due to a high recombination rate, leading to high luminescence rather than photocatalytic activity. The reactions that may be associated with photocatalysis are provided below.SC+hθ→SC(eCB−,hVB+)→RecombinationhVB+H2O→HO*+H+eCB−+Dissolved O2→O2−* eCB−+H+→H*H*+H*→H2 HO*+HO*→H2O+½O2 HO*+H*→H2O→Recombination
The photogenerated oxidizing species, i.e., hydroxyl radicals (OH*) and superoxide radicals (O2−*), may interact with contaminants, either biological or chemical, for mineralization, as depicted below,HO*+Contaminants→Oxidation→Mineralization (CO2+H2O)O2−*+Contaminants→Oxidation→Mineralization (CO2+H2O)
Among the existing photo catalysts, zinc oxide (ZnO) is an important competitor of TiO2 for environmental applications. ZnO has a sufficiently negative conduction band edge for superoxide radical formation from adsorbed oxygen, and a suitable valence band edge for hydroxyl radical formation from water oxidation. However, ZnO, by itself, suffers anodic photo corrosion under illumination and lacks sufficient stability in highly acidic and basic medium.
Thus, a sunlight active composite photocatalyst for water purification solving the aforementioned problems is desired.